1. Field of the Invention
The present invention relates to a warning system for an aircraft and more particularly to a warning system for an aircraft equipped with an autopilot system having a selectable barometric altitude hold and/or radio altitude hold function, which monitors the autopilot system and compares the barometric altitude and terrain clearance, respectively, of the aircraft with the preselected altitude hold values when the autopilot system is engaged and provides a warning when the actual terrain clearance or barometric altitude deviates from the selected values by a predetermined amount.
2. Description of the Prior Art
Autopilot systems for various types of aircraft are known in the art. These autopilot systems vary by aircraft type and are known to include various functions, including a radio altitude or terrain clearance hold (R/A HOLD) function and a barometric altitude hold (ALTITUDE HOLD) function. For example, an autopilot system for a Fokker 50 type aircraft, available from the Honeywell Corporation, includes an R/A HOLD function as well as an ALTITUDE HOLD function. The R/A HOLD function is normally used during low-altitude modes of flight to maintain the terrain clearance level when the autopilot is engaged and either of the R/A HOLD switches (also known as flight directors) is placed in a R/A HOLD mode. The ALTITUDE HOLD function maintains the current barometric altitude when the autopilot system is engaged and one of the ALTITUDE HOLD switches is in an ALTITUDE HOLD mode. The autopilot system provided for the Fokker 50 aircraft also includes a touch-control steering (TCS) system which enables the pilot of the aircraft to adjust the preselected terrain clearance or barometric altitude levels while the autopilot is engaged.
While the R/A HOLD and ALTITUDE HOLD functions are known to adequately maintain preselected terrain clearance or barometric altitude levels, aircraft have been known because of failure to deviate from the preselected levels while the autopilot system is engaged. Unfortunately, while an autopilot system is engaged, pilots are known to have a tendency to rely on the autopilot system. Thus, when the autopilot system is engaged and either an R/A HOLD or ALTITUDE HOLD function is selected, the pilot of an aircraft may be unaware of a hazardous flight condition resulting when the aircraft deviates from the preselected terrain clearance or barometric altitude levels.